Altimetric observation of wave attenuation through the Antarctic marginal ice zone using ICESat-2

The Antarctic marginal ice zone (MIZ) is a highly dynamic region where sea ice interacts with ocean surface waves generated in ice-free areas of the Southern Ocean. Improved large-scale (satellite-based) estimates of MIZ extent and variability are crucial for understanding atmosphere–ice–ocean interactions and biological processes and detection of change therein. Legacy methods for defining the MIZ are typically based on sea ice concentration thresholds and do not directly relate to the fundamental physical processes driving MIZ variability. To address this, new techniques have been developed to measure the spatial extent of significant wave height attenuation in sea ice from variations in Ice, Cloud and land Elevation Satellite-2 (ICESat-2) surface heights. The poleward wave penetration limit (boundary) is defined as the location where significant wave height attenuation equals the estimated error in significant wave height. Extensive automated and manual acceptance/rejection criteria are employed to ensure confidence in along-track wave penetration width estimates due to significant cloud contamination of ICESat-2 data or where wave attenuation is not observed. Analysis of 304 ICESat-2 tracks retrieved from four months of 2019 (February, May, September and December) reveals that sea-ice-concentration-derived MIZ width estimates are far narrower (by a factor of ∼ 7 on average) than those from the new technique presented here. These results suggest that indirect methods of MIZ estimation based on sea ice concentration are insufficient for representing physical processes that define the MIZ. Improved large-scale measurements of wave attenuation in the MIZ will play an important role in increasing our understanding of this complex sea ice zone

Biological responses to change in Antarctic sea ice habitats

Sea ice is a key habitat in the high latitude Southern Ocean and is predicted to change in its extent, thickness and duration in coming decades. The sea-ice cover is instrumental in mediating ocean–atmosphere exchanges and provides an important substrate for organisms from microbes and algae to predators. Antarctic krill, Euphausia superba, is reliant on sea ice during key phases of its life cycle, particularly during the larval stages, for food and refuge from their predators, while other small grazers, including copepods and amphipods, either live in the brine channel system or find food and shelter at the ice-water interface and in gaps between rafted ice blocks. Fish, such as the Antarctic silverfish Pleuragramma antarcticum, use platelet ice (loosely-formed frazil crystals) as an essential hatching and nursery ground. In this paper, we apply the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO) to review current knowledge about relationships between sea ice and associated primary production and secondary consumers, their status and the drivers of sea-ice change in this ocean. We then use qualitative network modelling to explore possible responses of lower trophic level sea-ice biota to different perturbations, including warming air and ocean temperatures, increased storminess and reduced annual sea-ice duration. This modelling shows that pelagic algae, copepods, krill and fish are likely to decrease in response to warming temperatures and reduced sea-ice duration, while salp populations will likely increase under conditions of reduced sea-ice duration and increased number of days of >0°C. Differences in responses to these pressures between the five MEASO sectors were also explored. Greater impacts of environmental pressures on ice-related biota occurring presently were found for the West and East Pacific sectors (notably the Ross Sea and western Antarctic Peninsula), with likely flow-on effects to the wider ecosystem. All sectors are expected to be impacted over coming decades. Finally, we highlight priorities for future sea ice biological research to address knowledge gaps in this field

The future of Arctic sea-ice biogeochemistry and ice-associated ecosystems

The Arctic sea-ice-scape is rapidly transforming. Increasing light penetration will initiate earlier seasonal primary production. This earlier growing season may be accompanied by an increase in ice algae and phytoplankton biomass, augmenting the emission of dimethylsulfide and capture of carbon dioxide. Secondary production may also increase on the shelves, although the loss of sea ice exacerbates the demise of sea-ice fauna, endemic fish and megafauna. Sea-ice loss may also deliver more methane to the atmosphere, but warmer ice may release fewer halogens, resulting in fewer ozone depletion events. The net changes in carbon drawdown are still highly uncertain. Despite large uncertainties in these assessments, we expect disruptive changes that warrant intensified long-term observations and modelling efforts

Biological responses to change in Antarctic sea ice habitats

Sea ice is a key habitat in the high latitude Southern Ocean and is predicted to change in its extent, thickness and duration in coming decades. The sea-ice cover is instrumental in mediating ocean–atmosphere exchanges and provides an important substrate for organisms from microbes and algae to predators. Antarctic krill, Euphausia superba, is reliant on sea ice during key phases of its life cycle, particularly during the larval stages, for food and refuge from their predators, while other small grazers, including copepods and amphipods, either live in the brine channel system or find food and shelter at the ice-water interface and in gaps between rafted ice blocks. Fish, such as the Antarctic silverfish Pleuragramma antarcticum, use platelet ice (loosely-formed frazil crystals) as an essential hatching and nursery ground. In this paper, we apply the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO) to review current knowledge about relationships between sea ice and associated primary production and secondary consumers, their status and the drivers of sea-ice change in this ocean. We then use qualitative network modelling to explore possible responses of lower trophic level sea-ice biota to different perturbations, including warming air and ocean temperatures, increased storminess and reduced annual sea-ice duration. This modelling shows that pelagic algae, copepods, krill and fish are likely to decrease in response to warming temperatures and reduced sea-ice duration, while salp populations will likely increase under conditions of reduced sea-ice duration and increased number of days of >0°C. Differences in responses to these pressures between the five MEASO sectors were also explored. Greater impacts of environmental pressures on ice-related biota occurring presently were found for the West and East Pacific sectors (notably the Ross Sea and western Antarctic Peninsula), with likely flow-on effects to the wider ecosystem. All sectors are expected to be impacted over coming decades. Finally, we highlight priorities for future sea ice biological research to address knowledge gaps in this field

Columnar Ice versus Platelet Ice: Differences, Consequences, and Significance

Antarctic land-fast sea ice (fast ice) is sea ice fastened to land or ice shelves. Fast ice is an important component of Antarctic coastal marine ecosystems, providing a prolific habitat for ice algal communities. Columnar ice is the usual mode of fast ice growth in relatively calm waters. However, near an ice shelf, pelagic ice crystals accumulate as an unconsolidated sub-ice platelet layer beneath the columnar ice (CI), where they are subsumed by the advancing sea ice interface to form incorporated platelet ice (PI). We have mapped the full crystallographic orientation of sea ice using electron backscatter diffraction (EBSD) to investigate the differences between CI and PI at the scale of ∼ 0.01 m. This is the first time EBSD has been used to study sea ice. Crystal preferred orientation in CI can be explained by ocean current as is well known from the literature. Analysis of misorientation between grains using EBSD data in PI indicates that the mechanical rotation of crystals at grain boundaries is the most likely explanation for preferred orientation in this case. We examine the consequences of the difference between CI and PI at the scale of ∼ 0.1 m. We demonstrate the feasibility of using temperature fluctuations as a proxy for fluid movement, a key process for supplying nutrients to Antarctic sea ice algal communities. CI and PI permeability distributions in the bottom 0.1 m of winter Antarctic sea ice are marginally different but their arithmetic means are both of order 10-9 m^2. We develop new observation-based algorithms to estimate Antarctic fast ice algal biomass and snow thickness from under-ice irradiance measurements. We analyse these high biomass measurements in CI and PI along transect lines (∼ km) at two contrasting fast ice sites, i.e., in McMurdo Sound and off Davis Station. These algorithms can be used for future non-invasive surveys for example by using moored sensors or underwater vehicles. Altogether the key message of this thesis is that we can apply the same parameterisation for CI and PI in thermodynamic sea ice models unless the crystal orientations are important. To demonstrate this we represent platelet ice processes in a one-dimensional model using the same permeability parameterisations for CI and PI. The results are in good agreement with observational data from an over-winter study in 2009 of McMurdo Sound. Ultimately, this model will improve our understanding of not only sea ice near ice shelves but also the biogeochemical significance of fast ice in Antarctic ecological system

Voronoi Dynamics Simulation of Platelet Sea Ice Growth with Diffusive Heat and Mass Transfer

Platelet ice is a sea ice type found near an ice shelf. Platelet crystals, which originate in the water column, rise to the surface and deposit under the sea ice cover in a loose layer forming a subice platelet layer. There they grow in the near-surface supercooled water to become frozen into the ice cover as incorporated platelet ice. Several Antarctic field campaigns have collected ice cores, measured crystallographic and physical properties and simultaneously recorded oceanographic conditions. However, some in situ measurements are difficult to acquire experimentally, in particular the solid fraction of subice platelet layer which is required in this region to obtain sea-ice thickness from remote-sensing measurements. Voronoi dynamics is a simple but efficient grain growth technique. By integrating this with mechanical stability and heat and mass transfer by diffusion, virtual ice cores are simulated in three dimensions. This model shows topological similarity with incorporated platelet ice from real sea ice cores. The calibrated spatial-temporal distributions of porosity, salinity, temperature and crystallographic c-axes are extracted and compared with the observations. The solid fraction of the subice platelet layer obtained from our simulation due to the local growth of platelet crystals within this layer is 0.22 ± 0.01. In order to account for the flux of new crystals deposited into the sub- ice platelet layer, this must be combined with a packing efficiency of the deposition of platelet crystals 0.06 ± 0.01 (Dempsey et al., 2010). The total solid fraction is 0.28 ± 0.01 which is in good agreement with 0.25 ± 0.06 reported by Gough et al. (2012)

Columnar Ice versus Platelet Ice: Differences, Consequences, and Significance

Antarctic land-fast sea ice (fast ice) is sea ice fastened to land or ice shelves. Fast ice is an important component of Antarctic coastal marine ecosystems, providing a prolific habitat for ice algal communities. Columnar ice is the usual mode of fast ice growth in relatively calm waters. However, near an ice shelf, pelagic ice crystals accumulate as an unconsolidated sub-ice platelet layer beneath the columnar ice (CI), where they are subsumed by the advancing sea ice interface to form incorporated platelet ice (PI). We have mapped the full crystallographic orientation of sea ice using electron backscatter diffraction (EBSD) to investigate the differences between CI and PI at the scale of ∼ 0.01 m. This is the first time EBSD has been used to study sea ice. Crystal preferred orientation in CI can be explained by ocean current as is well known from the literature. Analysis of misorientation between grains using EBSD data in PI indicates that the mechanical rotation of crystals at grain boundaries is the most likely explanation for preferred orientation in this case. We examine the consequences of the difference between CI and PI at the scale of ∼ 0.1 m. We demonstrate the feasibility of using temperature fluctuations as a proxy for fluid movement, a key process for supplying nutrients to Antarctic sea ice algal communities. CI and PI permeability distributions in the bottom 0.1 m of winter Antarctic sea ice are marginally different but their arithmetic means are both of order 10-9 m^2. We develop new observation-based algorithms to estimate Antarctic fast ice algal biomass and snow thickness from under-ice irradiance measurements. We analyse these high biomass measurements in CI and PI along transect lines (∼ km) at two contrasting fast ice sites, i.e., in McMurdo Sound and off Davis Station. These algorithms can be used for future non-invasive surveys for example by using moored sensors or underwater vehicles. Altogether the key message of this thesis is that we can apply the same parameterisation for CI and PI in thermodynamic sea ice models unless the crystal orientations are important. To demonstrate this we represent platelet ice processes in a one-dimensional model using the same permeability parameterisations for CI and PI. The results are in good agreement with observational data from an over-winter study in 2009 of McMurdo Sound. Ultimately, this model will improve our understanding of not only sea ice near ice shelves but also the biogeochemical significance of fast ice in Antarctic ecological system

Solidification effects of snowfall on sea-ice freeze-up: results from an onsite experimental study

Although the effects of snow during sea-ice growth have been investigated for sea ice which is thick enough to accommodate dry snow, those for thin sea ice have not been paid much attention due to the difficulty in observing them. Observations are complicated by the presence of slush and its subsequent freeze-up, and the surface heat budget might be sensitive to the additional ice thickness. An onsite short-term land fast sea-ice freeze-up experiment in the Saroma-ko Lagoon, Hokkaido, Japan was carried out to examine the effects of snowfall on the structure and surface heat budget of thin sea ice, based on observational results and a 1-D thermodynamic model. We found that snowfall contributes to the solidification of the surface slush layer, contributing ice thickness that is comparable to the snowfall amount and affecting the crystal texture significantly. On the other hand, the basal ice growth rate and turbulent heat flux were not significantly affected, being <3.1 x 10(-8) m s(-1) and 3 W m(-2), respectively. This finding may validate the omission in past studies of snow effect in estimating ice production rates in polynyas and has implications about the reconstruction of growth history from sample analysis

Effects of Snow and Remineralization Processes on Nutrient Distributions in Multi-Year Antarctic Landfast Sea Ice

We elucidated the effects of snow and remineralization processes on nutrient distributions in multi-year landfast sea ice (fast ice) in Lutzow-Holm Bay, East Antarctica. Based on sea-ice salinity, oxygen isotopic ratios, and thin section analyses, we found that the multi-year fast ice grew upward due to the year-by-year accumulation of snow. Compared to ice of seawater origin, nutrient concentrations in shallow fast ice were low due to replacement by clean and fresh snow. In deeper ice of seawater origin (the lower half of the multi-year fast ice column), remineralization was dominated by the degradation of organic matter. By comparison between first- and muti-year ice, the biological uptake and the remineralization were dominated in relatively young ice and older ice, respectively, under the physical process of brine drainage